1. Field of the Invention
The present invention relates to an image pickup apparatus and a method for controlling the same.
2. Description of the Related Art
Conventionally, a technology has been considered in which an image sensor including a plurality of photoelectric conversion units for a single microlens is used to obtain pupil-divided images, and a phase difference between the two obtained pupil-divided images is obtained, thereby performing focus detection (Japanese Patent Laid-Open No. 2001-250931).
In the technology of Japanese Patent Laid-Open No. 2001-250931, each photoelectric conversion unit has a reduced area relative to that of the conventional photoelectric conversion unit, conceivably causing a problem in that each photoelectric conversion unit has a reduced capacity to accumulate an electric charge and easily becomes saturated. If even one of the divided photoelectric conversion units becomes saturated, the relationship between amounts of incident light and output voltages will be nonlinear, even if the outputs of the plurality of photoelectric conversion units corresponding to the same microlens are added, resulting in a deterioration in image quality.
In order to address this problem, it is conceivable to adopt a pixel structure in which an electric charge leaks from the saturated photoelectric conversion unit into another photoelectric conversion unit that corresponds to the same microlens. With this, the electric charge that is output by the saturated photoelectric conversion unit is accumulated in the other unsaturated photoelectric conversion unit, enabling an increase in linearity between amounts of input light and output voltages obtained by adding the outputs of the plurality of photoelectric conversion units corresponding to the same microlens.
Although such a configuration makes it possible to suppress the deterioration in image quality caused due to loss of the electric charge, the saturation level of the individual photoelectric conversion units is not improved. Also, since the photoelectric conversion unit that accumulates the electric charge leaked from the saturated photoelectric conversion unit presents a higher output than the original output of the photoelectric conversion unit, the pupil-divided images used for obtaining the phase difference are deformed (image distortion occurs) and the accuracy of the phase difference (shift amount) detection is deteriorated. Specifically, the difference in outputs between the photoelectric conversion units may be reduced due to the leaked electric charge, and thus the difference between the pupil-divided images is also reduced, possibly causing pseudo in-focus in which shift is superficially reduced.
That is, the focus detection accuracy deteriorates and focus detection may not be possible. Since the focus detection accuracy deteriorates despite favorable images being obtained, no distinction is made between when focus detection is possible and when focus detection is not possible, which also causes the user confusion.
For example, by not using the output of pixels having a saturated photoelectric conversion unit for the phase-difference detection, the accuracy in phase-difference detection may be improved. However, in the case where the proportion of saturated photoelectric conversion units is high, normalization of a correlation image is necessary in order to compute an accurate shift amount, and because the calculation amount of normalization is proportional to the number of pixels included in the field of vision, the circuit size may increase following an increase in the number of pixels.